Control of cell divisions in the nervous system: Symmetry and asymmetry

The diverse cell types in the nervous system are derived from neural progenitor cells. Neural progenitors can undergo symmetric divisions to expand cell population or asymmetric divisions to generate diverse cell types. Furthermore, neural progenitors must exit the cell cycle in a developmentally regulated manner to allow for terminal differentiation. The patterns of neural progenitor divisions have been characterized in vertebrates and invertebrates. During the course of nervous system development, extrinsic and intrinsic cues dictate the division patterns of neural progenitors by influencing their cell cycle behavior and cellular polarity. The identification in Drosophila of asymmetrically distributed fate determinants, adapter molecules, and polarity organizing molecules that participate in asymmetric neural progenitor divisions should provide points of entry for studying similar asymmetric divisions in vertebrates.